A 30-yr-old woman with previously resected papillary thyroid carcinoma was found to have a pelvic lesion which concentrated radioiodine. By performing simultaneous 131I whole-body and 99mTc-methylene diphosphonate bone scans, we found the lesion to be in soft tissue between the sacrum and bladder. Radioiodine therapy was postponed so that the lesion, a benign teratoma of the rectal wall, could be surgically removed. Prior to laparotomy, the patient received a second tracer dose of 131I so that the lesion could be located at surgery with a hand-held gamma detector. A postoperative whole-body 131I scan confirmed that the lesion had been removed, thus reducing the absorbed radiation that would have been received by the ovaries during radioiodine therapy. Although the lesion contained both thyroid and gastric epithelium, accumulated 131I was limited to the area with thyroid follicles.